I think it is safe to say he doesn't have the same persona away from the camera but using those criteria David Letterman or Bill O'Reilly are characters.

No, Colbert is entirely adopting a new persona, like any actor. It's absolutely nothing like Letterman, who is simply being himself, and don't even compare him to Bill freaking O'Reilly. You guys watch WAY too much TV if you think that's his normal personality.